Printing processes typically involve the creation of one or more proofs that allow the printer or printing customer to view a sample of the image to determine if words are spelled correctly, if images are located as desired and to determine if colors have been reproduced satisfactorily. There are multiple points in the printing process where an image or composite of images may be critiqued. Proofing is a time-and-material consuming step in the printing process, but is traditionally considered to be necessary so that alterations and/or corrections, often at the last minute, can be made.
At first, proofs were made on the press used to print the job. Specifically, plates were made from composed films and one or more proofs were printed using the plate, ink and paper of the job. Often, several copies were printed for the approval of the customer. This proofing method is still the method of choice when large numbers of samples (referred to as "press proofs") are required. Such proofs are as close as possible to the output to be produced by the job.
Because of the length of the traditional proofing cycle, products were developed that could lead to substantial reduction in proofing time. Such proofing processes create "offpress" proofs by sequentially exposing the four color films onto photosensitive colored composite layers that simulate the printed sheet. The colorants in the photosensitive foils are pigments that are similar to the printing ink pigments. The four colored foils are laminated, and exposed through films, in register on a white receiver sheet to produce the final offpress proof. Typically, these proofs were made immediately after the color scanning process, after color scan corrections were made and then again for the final composite proof. Of course, every adjustment required another proof to show the effect of the changes. Typical examples of offpress proofs are Cromalin by DuPont and Matchprint and Transfer Key by Imation.
If only one or two copies of a proof are required, the offpress proof is less expensive than a press proof. While the spectral match to ink and paper may not be totally perfect, it is close enough to be a useful guide to color approval. The time required to prepare offpress proofs that are of contract quality and acceptance is approximately 25 to 45 minutes per proof, as compared to the two to four hour time required for press proofing. Until recently, this turnaround time was generally considered acceptable.
In the 1980's, desktop publishing became a reality. Computers were finally powerful and inexpensive enough to allow for prepress color preparation off-site (e.g., at the publisher) from the color trade shop and printing plant. The results of color scans were displayed on cathode ray tube (CRT) color monitors. Color corrections and image manipulation were performed on the color monitor in the red-green-blue phosphor color space and transformed into cyan-magenta-yellow-black (CMYK) printing plate files. The CMYK files were converted to films and offpress proofs or press proofs were made from the films. From the beginning, there was a desire to use the color monitor as a soft proofing medium, but there were some limitations. The customer would need a remote color monitor for simultaneous viewing of the proof. Further, the viewing conditions would have to be controlled and/or standardized. Also, the color reproduced by the monitor was not considered to be representative of ink on paper, and therefore not useful as a consistent guide to color proofing owing to manufacturing processes, inherent differences between reflective and self-luminous displays and the spectral mismatch between monitor primary colors and ink colors.
The demand for a soft proofing device has continued to increase, accelerated by other technological developments, primarily the "computer-to-plate" and "computer-to-press" printing processes. This technology has the potential to eliminate film completely, thus eliminating the offpress proof and increasing the demand for a high fidelity soft proofing system. Also, image assembly and page imposition in these processes are accomplished by manipulating data files, resulting in a significant reduction in the amount of time required to undertake these tasks and thereby shifting the production bottleneck to the proofing process.
Schreiber, U.S. Pat. No. 4,500,919 discloses a system for reproducing colors including apparatus for causing a reproduction on a CRT to be a colorimetric match for a final printed page. Changes to the image can be interactively made by observing the image on the CRT as adjustments are made.